The present invention relates to an electrophotographic process cartridge used in an elctrophotographic printing system such as a laser beam printer, a copying machine and a facsimile machine, and more particularly, to an electrophotographic process cartridge and a method for developing an electrostatic latent image with two layers or less of polymeric toner.
In general, a conventional electrophotographic process is performed using a photosensitive drum to obtain a hard copy by sequentially repeating a cycle of operations including charging, exposing, developing, transferring, cleaning and erasing, and fixing. This operation is referred to as the "Carlson process", and is disclosed in U.S. Pat. No. 2,297,691 issued to Carlson on Sep. 6, 1942. Further details of this process will be discussed later in this application.
One major concern in electrophotographic printing systems is toner waste. That is, during the electrophotographic printing process, only a portion of the total amount of developing material (i.e., toner) applied to a photosensitive element to effectuate the printing process is actually transferred onto a printable medium, such as paper. Accordingly, the portion of toner that is not transferred onto the printable medium remains as waste toner. Typically, the percentage of toner that is actually transferred during the electrophotographic printing process is referred to as the transfer rate. Obviously, it is quite desirable to maximize the transfer rate in an electrophotographic printing operation.
One recent effort that seeks to increase the transfer rate is disclosed in U.S. Pat. No. 5,422,708 entitled Apparatus And Method For Metering Toner In Laser Printers issued to Morris et al. on 6 Jun. 1995. In Morris et al. '708, a toner dispensing gap in a toner cartridge is set to within a particularly narrow range in an effort to thereby reduce the amount of toner that is released during the printing process. While this type of conventional art has some merit in its own right, we have discovered that this effort is premised upon its requirement for establishment and continued maintenance of an unnecessarily high degree of accuracy; consequently, we believe that an improved device can be contemplated.